Lee Elliott wrote: > On Friday 11 Nov 2005 02:47, Josh Babcock wrote: > >>Lee Elliott wrote: >> >>>On Thursday 10 Nov 2005 20:20, Andy Ross wrote: >>> >>>>After some prodding from Curt, I finally spent a few hours >>>>yesterday tracking down the "pitch down" discontinuity in >>>>the Citation. >>>> >>>>Well, I didn't find a discontinuity. I can now graph the >>>>lift curve from a Surface (a real one, part of the real >>>>aircraft, not an isolated test instance) and verify that >>>>it's valid and correct looking through the entire AoA >>>>regime. >>>> >>>>But I think I *did* find the problem: it seems that I, er, >>>>"misdocumented" the incidence and twist parameters in the >>>>YASim configuration. The README.yasim file states that >>>>these numbers are positive for positive AoA (i.e. a >>>>positive incidence on a wing generates extra lift, and a >>>>negative twist causes the wing tips to stall after the >>>>root). But the code was interpreting the number as a >>>>rotation about the YASim Y axis, which points out the left >>>>wing and therefore is positive *down*. Oops. >>>> >>>>The reason the citation exhibited this especially is just >>>>luck: the file lists an incidence of 3.0 (which is >>>>relatively high, and the inversion bug therefore puts the >>>>wing 3 degrees closer to a negative stall) the solver >>>>happens to generate a nose-down cruise configuration of >>>>about 1.5 degrees, and the elevator authority is actually >>>>quite high (which causes higher pitch rates under autopilot >>>>control). >>>> >>>>So the bottom line is that Curt was right: it *was* the >>>>negative AoA stall (probably the tail's, not the wing's) >>>>happening too soon. :) >>>> >>>>I'm a little leery of changing this in code this close to a >>>>release -- the risk of breaking working aircraft is too >>>>high. For the short term, this can be fixed in the >>>>Citation-II.xml file by simply negating the incidence and >>>>twist values on the wing. I did this and tried the >>>>autopilot in a maximum speed cruise at low level (which >>>>should produce the highest nose-down AoA) without any odd >>>>behavior. >>>> >>>>Curt, can you try that and see if it appears to fix the >>>>handling issues? Likewise, anyone with a YASim aircraft >>>>that makes use of incidence or twist values is encouraged >>>>to try the same modification and report any problems. We >>>>can go back after the release and fix the code and all the >>>>aircraft files. >>>> >>>>Andy >>> >>>I'll try to check the ones I've done over the weekend. The >>>one that concerns me most is the B-52F. The wing incidence >>>is set to 6 and the twist to -4 and I'm starting to wonder >>>how it manages to fly at all. >> >>Nose down. The fuselage is about 5 deg down when in level >>flight. >> >> >>>I got some good info on the B-52F from someone who flew >>>around 3000 hrs in that model and around 6000 hrs total in >>>all models, apart from the A/B, and it was flying to within >>>around 10 kts or so of what it should have been doing and >>>was climbing at about the right rate. >>> >>>LeeE > > > Depending on weight, alt and speed, 5 deg nose-down could be > correct. The incidence of +6 degrees is correct but I had to > estimate the twist. > > I should be able to have a look at it sometime this weekend. > > Ta for having a look. > > LeeE > > > _______________________________________________ > Flightgear-devel mailing list > Flightgear-devel@flightgear.org > http://mail.flightgear.org/mailman/listinfo/flightgear-devel > 2f585eeea02e2c79d7b1d8c4963bae2d >
Yeah, look at a picture of one in flight. The wings are mounted at a high AOA so it can make four point landings at low airspeeds and low descent rates. The b47 had a similar setup, but only the gear was level, the entire fuselage pointed up in the air on that one. Several soviet bombers with bicycle gear also had that look. Josh _______________________________________________ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
